Search Worksite documents with more than one parameters

I wrote a c# code to search worksite documents.  It works fine when I only provide one single parameter, however, if I provide more than one, it will return no result (result count=0).  I would like to search either (author=srchKey) OR (fulltext=srchKey).  I have been banging my head, could anyone please help? Thanks a lot.

Below is my code:

public List<iManageVO> RunSearch(string srchKey)
{

   try
   {

         iManageSearch rds = new iManageSearch(isession);

        // Populate searchparameters
        IManProfileSearchParameters searchparams = Utility.CreateUnpopulatedProfileParams(idms);
        

        // the code works, if there is only one searchparams, but if I un-comments one, it will return no result.

        // i would like to search either author = srchKey OR Description = srchKey
        //searchparams.Add(imProfileAttributeID.imProfileAuthor, srchKey);
        //searchparams.Add(imProfileAttributeID.imProfileFullText, srchKey);
        searchparams.Add(imProfileAttributeID.imProfileDocNum, srchKey);
        //searchparams.Add(imProfileAttributeID.imProfileDescription, srchKey);

       // Search documents
        IManDocuments results = rds.GetDocuments(Utility.BuildDatabaseList(isession.Databases), searchparams);

        List<iManageVO> foundDocuments = null;
        // Check results
        if (results != null)
         {
                foundDocuments = new List<iManageVO>();
                strResult = "Search returned " + results.Count + " document(s).";
                foreach (IManDocument document in results)
                  {
                        
                                 iManageVO doc = new iManageVO();
                                 doc.Description = document.Description;
                                 doc.Author = document.Author.FullName;
                                 doc.DocId = document.Number;

                                  doc.iManageNrl = document.ObjectID;

                                  foundDocuments.Add(doc);
                             
                     }

             }

               return foundDocuments;
    }
    catch (ApplicationException ae)
    {
                 throw ae;
     }


}